In wireless broadcast systems where one or more antennas broadcast signals toward wireless transmit and receive units (WTRUs), there are sometimes null areas where interfering signals interact in a fashion that a WTRU in a certain location cannot decode the received signal. Null areas may be caused by a single antenna whose signal is subject to multipath wherein a direct path signal overlaps with one or more reflected signals. Null areas may also be caused by a plurality of antennas whose signals overlap.
As a result of null areas being present in a region of overlapping signals, while the majority of intended WTRUs in such overlapping regions may be able to properly receive signals, other WTRUs might not. Some WTRUs might not be able to decode the signals, and depending on the nature of the broadcast, there may be consequences of lost communication or other interference. Yet other intended WTRUs might interrogate the system later to see if they had missed some message, and if so, request retransmission of the message uniquely to themselves. Increasing the number of retransmissions results in less air time available for other transmissions. There may also be a timeliness issue about the delivery of the message. One method of addressing the problem in the prior art is to use time-diversity, thus reducing the odds that a WTRU would be in the null area for alternative directions of transmission or time frames.
U.S. Pat. No. 6,061,574 to Goldberg, teaches a controller which controls at least two transmitters to transmit simulcast signals during a time period. The two simulcast signals produce intersymbol interference at a receiver. One of the transmitters changes its output amplitude during a portion of the time period, altering the symbol interference during a portion of the time period.
U.S. Pat. No. 6,044,276 to Goldberg, et al. teaches a messaging system using a plurality of simulcasting base transmitters and a plurality of pseudorandom sequence generators. The generators generate pseudorandom sequences, which are different from one another during concurrent transmission by the base transmitter. A cancellation-affecting parameter of the plurality of base transmitters is adjusted in accordance with the plurality of pseudorandom sequences during the transmission from the base transmitters to limit intervals of carrier cancellation.
Known prior art does not offer a simple and inexpensive solution for addressing signal interference caused in the overlapping region of the signal beams from one or more antennas. Prior art techniques are generally consumers of radio frequency resources and are therefore undesirable. Prior art techniques also delay the time at which the communication is received by a certain percentage of the WTRUs or signal recipients.
It is therefore desirable to address null areas, without substantially increased cost and with a minimum consumption of RF resources.